More than 35 years into their mission, our farthest-flung spacecraft are not finished yet.

At Mission’s End The Voyagers will eventually “go quiet” around 2025, when their power source—plutonium 238—has decayed to the point that transmissions end. Five years before that, scientists will have to begin turning off instruments in order to conserve signal power. “Plutonium 238 has an 88-year half-life, so we can predict when we’ll have to turn off the first instrument,” says Stone. Now that the Voyagers have reached escape velocity, they’re never coming back to the solar system. “I’ll be a little sad to see them go,” says Christian. “But it’s neat that this is mankind’s step to the stars, like a message in a bottle. This is one thing that NASA has done fantastically on, and the public has really responded. It’s real exploration.”